Optical devices (e.g., light emitting diodes and semiconductor lasers) and high performance semiconductor devices (e.g., heterojunction field-effect transistors and heterojunction bipolar transistors) have therein semiconductor material with different bandgap energies. The different bandgap energies, and associated refractive index differences, provide the desired quantum mechanical features and functions in the devices, such as potential wells, tunneling and waveguiding. Compound semiconductors, such as gallium arsenide (GaAs), provide the desired different bandgap energies and are easily deposited in precisely controlled layers. For example, by substituting aluminum for gallium in gallium arsenide (forming aluminum gallium arsenide or AlGaAs), the bandgap energy can be increased. Generally, the more the aluminum added, the larger the bandgap energy. The GaAs/AlGaAs materials can be deposited in layers using solid source molecular beam epitaxy (MBE), metal-organic MBE, gas source MBE or metal-organic chemical vapor deposition (MOCVD). These two techniques allow for almost atom-by-atom growth of the compound semiconductor crystal.
To form the optical devices and high performance semiconductor devices out of the layers of compound semiconductor material, it is frequently necessary to selectively etch adjacent layers of material which differ only in their bandgap energies. For GaAs/AlGaAs, commonly used etchants etch the GaAs and AlGaAs at rates depending on the amount of aluminum in the AlGaAs. For example, K. Kenefick in "Selective Etching Characteristics of Peroxide/Ammonium-Hydroxide Solutions for GaAs/Al.sub.0.16 Ga.sub.0.84 As", Journal of the Electrochemical Society, Vol. 129, No. 10, pp. 2380-2382, discloses a hydrogen peroxide (H.sub.2 O.sub.2)/ammonium hydroxide (NH.sub.4 OH) etchant with a selectivity of etching Al.sub.0.16 Ga.sub.0.84 As over GaAs of 10 to 30 depending on the pH of the H.sub.2 O.sub.2 /NH.sub.4 OH solution. Another etchant is described by X. S. Wu et al. in "Selective Etching Characteristics of HF for Al.sub.x Ga.sub.1-x As/GaAs", in Electron Letters, Vol. 21, pp 558-559. Here, hydrofluoric acid (HF) is used to etch AlGaAs at relatively high temperatures (80.degree. C.) using a gold mask to protect the underlying AlGaAs where no etching is desired.